Coronary heart disease and other forms of atherosclerosis are the greatest cause of morbidity and mortality in the Western world. The risk for developing atherosclerosis increases as the total serum cholesterol increases. Elevation of VLDL, LDL, and their major protein, apoprotein B, has been associated with an increased risk of atherosclerosis, whereas elevation of HDL has been associated with a decreased risk of this disease. Lipoprotein metabolism can be conveniently, yet intensively, studied by injecting radiolabeled HDL, LDL, or VLDL into the blood stream and then carefully measuring its disappearance from the blood, along with the excretion of radioactive iodine as a breakdown product in the urine. These studies can extend our understanding of lipoprotein metabolism in directions that may provide a basis for more effective treatment, or even prevention, of atherosclerosis. Questions to be addressed in Lipoprotein Turnover studies include: 1. How does the synthesis rate and interconversion of lipoproteins change with different dietary manipulations? 2. What are the main determinants of HDL levels? What is the heritability of the HDL metabolic parameters? 3. What are the similarities and differences in HDL metabolism between normal subjects and patients with unusually high or low HDL levels, or patients with atherosclerosis unexplained by known risk factors? 4. Why are the plasma levels of HDL in men and women different, and could this relate to men's increased risk of atherosclerosis? 5. How does the percent conversion of VLDL to LDL change with different dietary manipulations? 6. When body composition studies are performed in identical and fraternal twins, what is the inheritance of bone mineral density?. 7. What is the relationship of body composition and HDL turnover parameters? We are recruiting identical (MZ) and fraternal (DZ) twins for these studies to estimate the extent of genetic control of many different phenotypes in humans. In twin studies, the difference in pair similarity between MZ and DZ twins indicates how much of the phenotype in genetically controlled. We aim to study lipoprotein levels and related parameters in MZ and DZ pairs on low and high fat diets under metabolic ward conditions to estimate the genetic control of lipoprotein levels on a fixed diet and the genetic control of diet response. Correlation of body composition studies to lipoprotein metabolic parameters are also under study, as it might explain in part the differences in lipoprotein levels not accounted for the genetic background. Body composition studies include the use of DPX (Dual photon absorptiometry), which measures body bone mass and whole body bone density utilizing an x-ray source adding information on fat distribution.